The present invention relates to a method for making alkali metal phenoxide salts. More particularly, the present invention relates to a method for making aromatic ether imides based on the use of an alkali metal phenoxide salt obtained by azeotroping a water-hydrocarbon solvent mixture from an aqueous mixture of an in situ formed alkali metal phenoxide salt.
Prior to the present invention, as shown by Wirth et al U.S. Pat. No. 3,787,364, assigned to the same assignee as the present invention, alkali metal phenoxide salts were made by effecting reaction between alkali metal hydroxides and bisphenols in the presence of water, where the water was removed by azeotropic distillation using a hydrocarbon solvent and a dipolar aprotic solvent. Although this procedure was effective for making polyetherimides, it could not be used without the presence of the dipolar aprotic solvent which rendered the procedure economically less attractive. Improved results were achieved when the bisphenoxide salt was made in accordance with the procedure of White U.S. Pat. No. 3,852,242, assigned to the same assignee as the present invention, based on the use of an alkali metal alkoxide which was reacted with a bisphenol to produce the alkali metal bisphenoxide in the absence of water. An additional procedure is shown by copending application of Tohru Takekoshi, RD-10745, based on the rapid separation of water, such as by flashing from a hydrated alkali metal bisphenoxide, or an aqueous slurry containing stoichiometric amounts of alkali metal hydroxide and the bisphenol. The advantages of the procedure of Takekoshi is that elevated temperatures can be used, for example, above 100.degree. C. and as high as 300.degree. C. to effect the rapid separation of water from aqueous bisphenoxide alkali metal hydroxide slurry, or the hydrate of the alkali metal bisphenoxide salt to produce a substantially anhydrous bisphenoxide salt. Although the procedure of Takekoshi also is effective for making substantially anhydrous alkali metal bisphenoxides, special equipment, such as spray dryers, drum dryers, etc., are needed to render the procedure economically feasible.
The present invention is based on the discovery that unlike the procedures of the prior art requiring either expensive dipolar aprotic solvents, special equipment or the synthesis of alkali metal alkoxides, substantially anhydrous alkali metal bisphenoxides can be made by initially preparing a homogeneous solution of an aqueous metal hydroxide and a bisphenol and thereafter combining the aforementioned aqueous bisphenoxide solution with a substantially inert hydrocarbon solvent to produce a heterogenous mixture which is thereafter refluxed to effect the separation of water by azeotropic distillation. The surprising feature of the present invention is that unless the initial mixture of alkali metal hydroxide and bisphenol is in the form of a homogeneous solution, effective removal of water cannot be achieved and the resulting alkali metal bisphenoxide salt does not provide for satisfactory yields of aromatic ether imide when used in reaction with substituted phthalimides, such as nitro phthalimide.